1. Field of the Invention
This invention relates to an electric discharge contour machining method for machining a workpiece with a simple electrode.
2. Description of the Prior Art
FIG. 4 is an overall view showing a structure of a conventional electric discharge machining apparatus.
In FIG. 4, reference numeral 1 denotes an electric discharge machining electrode 2 a workpiece to be machined, 3 a machining tank, and 4 machining solution such as insulation oil stored in the machining tank 3. Machining by the electric discharge machining apparatus is normally conducted in the machining solution 4 in the machining tank 3. Reference numeral 5 denotes a current generator for supplying an electric discharge pulse current to the electrode 1 and the workpiece 2, 8a a ball screw for moving a main spindle of the electric discharge machining apparatus in vertical directions such as upward and downward directions (Z-axis directions), 8c a ball screw for moving the workpiece 2 in lateral directions such as rightward and leftward directions (X-axis directions), 9a a servo motor for rotatably driving the Z-axis ball screw 8a, 9b a servo motor for rotatably driving a ball screw (not shown) for moving the workpiece 2 in longitudinal directions such as forward and reverse directions (Y-axis directions), 9c a servo motor for rotatably driving the X-axis ball screw 8c, and 10 a numerical controller for suitably controlling the drives of the servo motors 9a, 9b and 9c in response to a machining program. Reference numeral 11 denotes a head disposed at the top of the electric discharge machining apparatus, 12 a column of a frame for fixedly supporting the head 11, 13 a bed of a foundation of this electric discharge machining apparatus, 14b a movable table for moving the workpiece 2 together with the machining tank 3 in the longitudinal directions (Y-axis directions), 14c a movable table for moving the workpiece 2 together with the machining tank 3 in the lateral directions (X-axis directions), and 15 a spindle head of the main spindle of the electric discharge machining apparatus.
The conventional electric discharge machining apparatus is constructed as described above, and the workpiece 2 is electric discharge-machined by the electrode.
In the electric discharge machining operation, both the electrode 1 and the workpiece 2 are dipped in the machining solution 4 in the machining tank 3, and a pulse current is supplied from the pulse current generator to the electrode 1 and the workpiece 2. An intermittent electric discharge is generated in a machining gap between the electrode 1 and the workpiece 2 by the supply of the pulse current, and the workpiece 2 is machined by the electric discharge. In this case, since the electrode 1 is coupled to the Z-axis servo motor 9a via the ball screw 8a, the electrode 1 is moved vertically upward or downward in response to a command from the numerical controller 10. The Y-axis servo motor 9b is connected to the Y-axis movable table 14b via a ball screw (not shown), and the X-axis servo motor 9c is connected to the X-axis movable table 14c via the ball screw 8c. These movable tables 14b and 14c are suitably moved longitudinally and laterally in response to the command from the numerical controller 10. Thus, horizontal relative positional displacements of the electrode 1 and the workpiece 2 such as horizontal positioning or lateral machining, etc., can be arbitrarily altered. Therefore, the arbitrary position of the workpiece 2 can be electric discharge-machined in an arbitrary shape by adequately controlling the drives of the servo motors 9a, 9b and 9c.
The workpiece 2 may be contour-machined. As shown in FIGS. 5(a) and 5(b), a rod-shaped electrode 1 of a circular cross-section is attached to the spindle head 15, and the contour of the workpiece 2 is machined while continuously rotating the electrode 1. The machining shape is applied in advance to the numerical controller 10 by a program and the controller will determine the movement of the electrode with respect to the workpiece. The workpiece before machining and the programmed path of the electrode is shown in FIG. 5(a), and the workpiece shape after machining is shown in FIG. 5(b).
During the machining of a contour, the profile of the electrode 1 may become distorted, as shown in FIG. 6(a). Thus, even through one end of the electrode 1 remains attached to the spindle head 15, the remainder of the electrode is bent and presents a variable discharge gap to the surface of the workpiece being machined. At its opposite, unattached end, the electrode profile may deviate from the desired location by an amount .DELTA.. Since the outer periphery of the electrode 1 is made of copper, tungsten, or similar conductive material, the electrode can have its shape maintained by machining. For example, an electrode correcting material 100 may be disposed in parallel with the desired electrode profile and used to correct the deviation. The deviation is corrected by electric discharge machining (EDM) of the rod surface so that the profile of the rod-shaped electrode 1 is parallel to the rotational axis of the spindle 15, as shown in FIG. 6(b).
While the EDM processing of the electrode will maintain a desired electrode profile, the outer diameter of rod will be varied along its length. This could be significant to the effective operation of the electrode in machining a desired contour. For example, the outer diameter of the rod-shaped electrode is determined on the basis of the smallest portion of the inner corner R of the contour machining shape of the workpiece 2. If that outer diameter is changed, the machining accuracy may be affected.
Also, since the conventional contour machining method is arranged as described above so that the electrode of the outer diameter is coincident with the smallest inner corner R of the contour machining shape of the workpiece, there arises a problem in that it may take a long machining time to machine large contours. This problem is due to the fact that the outer diameter of the electrode has been restricted to a small value due to the machining requirements for the inner corner R.
It is an object of the present invention to solve the above-described problem. It is a further object of the invention to provide an electric discharge contour machining method which can efficiently machine a variety of contours without being restricted by the dimensions of the smallest inner corner contour to be machined in a workpiece.